Shell and tube heat exchanger, finned tubes for such heat exchanger and corresponding method

ABSTRACT

A shell and tube longitudinal flow heat exchanger comprising a containment casing 101 within which a first fluid can flow substantially parallel to the longitudinal axis of said casing 101, said containment casing 101 accommodating in its interior a bundle of tubes 2 substantially parallel to one another and parallel to the longitudinal axis of said casing 101 and a plurality of grid-shaped baffles 102 substantially transverse to the longitudinal axis of said casing 101 supporting said tubes 2, a second fluid flowing in said bundle of tubes 2. Said tubes 2 are provided on at least a part of their outside surface with a plurality of low fins 21, which are helically arranged on the outer surface of said tubes 2 with a first angle of advancement α and having a profile interrupted by helical grooves 22 having a second angle of advancement β, with α≠β.

The present invention relates to a shell and tube heat exchanger, in particular to a shell and tube heat exchanger comprising finned tubes of particular type. In a further aspect, the present invention relates to a method for manufacturing finned tubes provided with a particular system of fins.

Heat exchangers of shell and tube type are industrial heat exchangers of known type and consist essentially of a bundle of tubes positioned inside a containment casing (shell), usually cylindrical. In operating conditions two fluids flow through the heat exchanger: a first fluid, preferably the hotter, or more corrosive, or with higher fouling coefficient, flows inside the tubes (“tube side” flow), while a second fluid flows in the space delimited by the internal surface of the shell and by the outer surfaces of the tubes (“shell side” flow).

Transverse baffles (“diaphragms”), generally made of sheet metal, are generally present inside the shell with the dual purpose of supporting the bundle of tubes and generating turbulence in the fluid on the shell side in order to increase the heat transfer coefficient.

With reference to FIG. 3, the transverse baffles are made of sheet metal plates that occupy a part of the internal section of the heat exchanger 10, producing a tortuous path (as represented by the arrows in FIG. 3) of the fluid on the shell side, having longitudinal and above all transverse components with respect to the axis of the heat exchanger 10; this type of diaphragm configures the conventional solution classified according the TEMA standard commonly adopted at international level.

In longitudinal heat exchangers, an example of which, of the EMBaffle® (Expanded Metal Baffle) type, is represented in the accompanying FIGS. 1 and 2, the fluid on the shell side flows in a substantially rectilinear direction (for example, as represented by the arrows in FIG. 2)—generally in counterflow to the fluid on the tube side—and substantially parallel to the axis of the heat exchanger 1.

The flow of fluid on the shell side can also have a helical trend, for example as represented in FIG. 4. In this case, a certain number of diaphragms, made of stamped grids, are positioned inside the heat exchanger 100 and inclined so as to set the fluid on the shell side in rotary motion during its advancement through the heat exchanger 100, thereby producing the overall helical motion of the fluid on the shell side, schematized by the arrows of FIG. 4.

A typical problem that is encountered in heat exchangers of conventional TEMA type is given by the deposit of solid material transported by the fluid, or which forms by precipitation on the diaphragms or dead corners in the path of the fluid on the shell side. The deposit of solid material can cause a decrease in the heat transfer coefficient resulting in a decrease in the performance of the heat exchanger. Moreover, the presence of solid material deposited inside the heat exchanger can cause uneven distribution of the flow of fluid on the shell side, and can therefore lead to worsening in the performance of the heat exchanger.

In the field of heat exchangers, in particular heat exchangers of industrial type, there is known the use of tubes provided with surface fins to increase the heat exchange surface. In the case of shell and tube heat exchangers, depending on whether the performance is to be increased on the shell side or on the tube side, the tube can be provided with fins on its outer surface or on its internal surface. In particular cases, a finned tube is used on both surfaces.

In conventional heat exchangers (with transverse main flow of the type represented in FIG. 3), the fins normally used are transverse to the tube, so as to maximize heat exchange with the main component of the flow of fluid on the shell side. In a shell and tube longitudinal flow heat exchanger (of the type represented in FIGS. 1 and 2) these transverse fins (i.e. with angle of advancement α=90) would instead lose efficiency.

There are also known tubes provided with helical fins, or tubes in which the angle of advancement of the fin has a component in longitudinal direction with respect to the axis of the tube (α<90°).

The currently known methods for manufacturing tubes with helical fins employ a combination of several tools to maximize the number of helical fins. However, known technologies for manufacturing finned tubes suffer from a limitation in their application, depending on the characteristics of the material with which the tube is made, thereby limiting the range of materials on which fins can be produced.

In fact, in the presence of alloy tubes with higher mechanical strength (i.e. stainless and duplex steels), the longitudinal force component acting on the tube during formation of the fin, not evenly distributable between the tools, causes, due to progressive hardening of the material caused by the action of the tools in succession, systematic slipping of the tool with the greatest load outside the profile during manufacture. This results in damage to the tool and the need to frequently replace it, with the resulting damages in terms of direct cost of the tool and lack of production due to machine downtime.

To minimize the hardening effect that makes it difficult, if not impossible, to obtain given heights of the fin in the presence or alloy steels (stainless and higher) the tube is normally subjected to annealing between two subsequent machining operations, with a noteworthy increase in the costs of the production process of the tube.

Based on these considerations, the main aim of the present invention is to provide a shell and tube heat exchanger that solves the drawbacks and problems described above.

Within this aim, an object of the present invention is to provide a shell and tube longitudinal flow heat exchanger with improved performance with respect to heat exchangers of known type.

Another object of the present invention is to provide a shell and tube longitudinal flow heat exchanger in which the heat transfer coefficient per unit of pressure drop is increased, in particular on the shell side, with respect to conventional heat exchangers.

Yet another object of the present invention is to provide a shell and tube longitudinal flow heat exchanger in which the heat transfer coefficient per unit of pressure drop is increased, both on the shell side and on the tube side, with respect to conventional heat exchangers.

A further object of the present invention is to provide a finned tube for heat exchangers, in particular for tube and shell heat exchangers, that enables the performance of the heat exchanger to be improved.

Another object of the present invention is to provide a method for manufacturing finned tubes that allows the manufacture of tubes provided with helical fins (α<90°) even when said tubes are made of materials with high mechanical strength.

One more object of the subject matter of the present invention is to provide a shell and tube longitudinal flow heat exchanger, and a finned tube for heat exchangers, that is highly reliable and is easy to manufacture at competitive costs.

This aim, and these and other objects that will be more apparent below, are achieved with a shell and tube longitudinal flow heat exchanger comprising a containment casing within which a first fluid can flow substantially parallel to the longitudinal axis of said casing; said containment casing accommodates in its interior a bundle of tubes substantially parallel to one another and parallel to the longitudinal axis of said casing and a plurality of grid-shaped baffles substantially transverse to the longitudinal axis of said casing supporting said tubes, said bundle of tubes being adapted to the flow of a second fluid therein; the heat exchanger according to the present invention is characterized in that said tubes are provided on at least a part of their outside surface with a plurality of low fins, said low fins being helically arranged on the outer surface of said tubes with a first angle of advancement α and having a profile interrupted by helical grooves having a second angle of advancement β, with α≠β. In fact, it has been noted that a shell and tube longitudinal flow heat exchanger thus conceived has a series of characteristics and properties that allow the drawbacks and problems described above to be solved.

In particular, it has been seen that the presence of helical fins allows a substantial increase in the heat transfer coefficient on the shell side, thereby improving the performance of the heat exchanger.

The presence of breaks or interruptions on the profile of the fin, due to the helical grooves produced as described below, allows the creation of a three-dimensional surface that further increases the heat transfer area with respect to the initial fin. The final surface obtained is greater by a factor of 3.0-4.0, and can even reach a factor of 4.5, with respect to the initial smooth tube.

Moreover, as better explained below, due to the particular method for manufacturing the finned tubes, said method also forming the subject-matter of the present invention, it is possible to use in the heat exchangers of the present invention finned tubes made of materials with high mechanical strength, for example alloy steels, such as copper-nickel, stainless, duplex or titanium steels, which are critical for the reasons set forth above.

In fact, to date there are no known shell and tube longitudinal flow heat exchangers provided with tubes, in particular tubes made of materials with high mechanical strength, on the outer surface of which low helical fins are arranged.

For the objects of the present invention, the term “low fins” is intended as fins with a height H of less than approximately 2 mm and preferably between 0.5 and 1.5 mm.

The angle of advancement α of the fins is generally <80°, and preferably 15°≤α≤60°, more preferably 20°≤α≤45°, this latter being the best range to obtain the optimum compromise between fin height and density, as can be experimentally verified.

As better described below, the interruptions on the profile of the fin can be obtained by subjecting the tube to two finning operations in immediate sequence carried out with different angles of advancement.

The tube is subjected, with a first finning/grooving tool, to a first groove machining operation that produces a fin with an angle of advancement β, of low depth, preferably ≤0.5 mm, to limit hardening of the material. A second main finning operation is carried out on the tube thus grooved to produce the actual fins with an angle of advancement α.

In this way, the main fins are produced on a surface that already has ridges and grooves. The method uses angles of advancement and pitches for the first finning/grooving tool and for the main finning tool such that the result of machining of the main tool is to increase the height of the finished fin with respect to what can be obtained starting from a normal circular smooth surface.

In particular, the main fin machining operation takes place according to a plane inclined by an angle α₂ with respect to the longitudinal axis of the tube, while the fin/groove machining operation takes place according to plane inclined by an angle α₁ with respect to the longitudinal axis of the tube. The relative angle between the two machining planes (rake angle) is chosen on the basis of a compromise between the largest obtainable increase in the height of the fin and the largest number of interruptions obtainable per unit of length measured according to the longitudinal axis of the tube.

The rake angle is therefore between 0° (maximum height increase and no interruption) and 90° (minimum height increase and maximum interruption effect). Preferably, the rake angle is between 30 and 60°, according to needs. In this way, both the increase in height of the final fins with respect to the single machining operation and, at the same time, the desired interruption are obtained.

The fact that the two groove/fin machining operations are carried out almost simultaneously allows the effect of hardening to be minimized, which would otherwise make it very difficult to obtain the result of increase in the height of the fins with alloy steels (stainless and higher), without subjecting the tube to annealing between two subsequent machining operations as mentioned above, with the consequent increase in costs.

In the shell and tube heat exchanger according to the present invention, the relative angle between said first angle of advancement α and said second angle of advancement β is preferably between 0° and 90°, and more preferably between 30° and 60°.

The interrupted fin thus obtainable can extend for the whole surface of the tube or for portions of any length, leaving the remaining portions smooth. This characteristic is useful when using tubes with U-shaped bends, so as not to weaken the curved section, retaining their mechanical strength in particular applications.

In the case of shell and tube longitudinal flow heat exchangers of EMbaffle® type, this characteristic is particularly useful as the smooth portion facilitates stable positioning of the baffle; for this reason, shell and tube longitudinal flow heat exchangers according to the present invention can advantageously be provided with tubes in which finned portions are alternated by smooth portions.

In order to improve the heat transfer coefficient also on the tube side, the shell and tube heat exchanger according to the present invention is advantageously provided with tubes that can be equipped with interior fins obtained by producing grooves on the internal surface.

In a further aspect thereof, the present invention also relates to a method for manufacturing a finned tube using a machine comprising a first working assembly and at least one support assembly. The first working assembly comprises a first rotating finning/grooving tool and a second rotating finning tool mounted in sequence on the same driving axis. The first rotating finning/grooving tool is provided with a first helical working profile having a first angle of advancement α₁, while the second rotating finning tool is provided with a second helical working profile having a second angle of advancement α₂, with α₂≠α₁.

The method according to the present invention comprises advancing said tube on a plane defined by said support assembly, forming a first (temporary) fin/groove on said tube by means of said first rotating tool, forming a second (main) fin on said tube by means of said second rotating tool, the formation of said second fin being immediately subsequent to the formation of said first fin; moreover, the height of said first fin is generally lower than the height of said second fin.

In the more frequent case in which the driving axis of the first rotating finning/grooving tool and of the second rotating finning tool is parallel to the longitudinal axis of the tube, the first angle of advancement α₁ will have the same value as the second angle of advancement β of said first (temporary) fin/groove, and the second angle of advancement α₂ will have the same value as the first angle of advancement α of said second (main) fin.

As explained previously, the relative angle (rake angle) between said first angle of advancement α₁ and said second angle of advancement α₂ is advantageously between 0° and 90°, and preferably between 30° and 60°, Moreover, the first rotating finning/grooving tool and the second rotating finning tool are advantageously shaped so that the height h of said first fin is preferably ≤0.5 mm and the height H of said second fin is preferably ≤2 mm.

A finned tube for heat exchangers, in particular for shell and tube heat exchangers, obtained using the method described herein also forms the subject matter of the present invention In particular, the finned tubes of the present invention are provided on at least a part of their outer surface with a plurality of low fins, which are helically arranged on the outer surface of said tube with a first angle of advancement α and have a profile interrupted by helical grooves having a second angle of advancement β, with α≠β, said angle of advancement α being preferably <80°, and more preferably 150≤α≤60°, the relative angle between said first angle of advancement α and said second angle of advancement β being preferably between 0° and 90°, and more preferably between 30° and 60°, said low fins having a height H preferably ≤2 mm and more preferably between 0.5 and 1.5 mm.

Further characteristics and advantages of the present invention will be more apparent from the description of preferred, but not exclusive, embodiments of a shell and tube longitudinal flow heat exchanger according to the present invention, illustrated by way of non-limiting example in the accompanying drawings, wherein:

FIG. 1 shows a perspective view of a shell and tube longitudinal flow heat exchanger:

FIG. 2 shows a schematic view of a shell and tube longitudinal flow heat exchanger:

FIG. 3 shows a schematic view of a shell and tube tortuous flow heat exchanger:

FIG. 4 shows a schematic view of a shell and tube helical flow heat exchanger;

FIG. 5 shows a portion of finned tube that can be used in a shell and tube longitudinal flow heat exchanger according to the present invention:

FIG. 6a schematically shows the helical trend of the fins of a finned tube that can be used in a shell and tube longitudinal flow heat exchanger according to the present invention;

FIG. 6b schematically shows the helical trend of the grooves that interrupt the profile of the fins of a finned tube that can be used in a shell and tube longitudinal flow heat exchanger according to the present invention;

FIGS. 7a-7c show sections of alternative profiles of fins of a finned tube that can be used in a shell and tube longitudinal flow heat exchanger according to the present invention;

FIG. 8 shows a portion of finned tube with finned portions alternated by smooth portions that can be used in a shell and tube longitudinal flow heat exchanger according to the present invention;

FIG. 9 is a schematic side view of a first embodiment of a machine for implementing the method for manufacturing finned tubes according to the present invention;

FIG. 10 is a schematic front view of the machine of FIG. 9;

FIG. 11a schematically illustrates the formation of a first fin/groove on a tube with the method for manufacturing finned tubes according to the present invention;

FIG. 11b schematically illustrates the formation of a second (main) fin on a tube with the method for manufacturing finned tubes according to the present invention:

FIG. 12 is schematic side view of a second embodiment of a machine for implementing the method for manufacturing finned tubes according to the present invention:

FIG. 13 is a schematic front view of the machine of FIG. 12;

FIG. 14 shows a schematic view of a shell and tube longitudinal flow heat exchanger according to the present invention;

FIG. 15 shows a detail of the shell and tube longitudinal flow heat exchanger of FIG. 14.

With reference to the appended figures, a shell and tube longitudinal flow heat exchanger of EMBaffle® type comprises, in its more general embodiment, a containment casing 101 within which a first fluid can flow substantially parallel to the longitudinal axis of said casing 101. Inside the containment casing 101 there is positioned a bundle of tubes 2, which are substantially parallel to one another and parallel to the longitudinal axis of the casing 101; the casing 101 also contains a plurality of grid-shaped baffles 102 transverse to the longitudinal axis of said casing 101, said baffles 102 supporting said tubes 2.

With particular reference to FIG. 14, a second fluid flows inside the tubes 2, generally in counterflow (see arrows 210) to the direction of flow of the first fluid inside the shell 101 (see arrows 110).

With reference to FIG. 7, one of the peculiar characteristics of the shell and tube longitudinal flow heat exchanger 1 according to the present invention is given by the fact that said tubes 2 are provided on at least a part of their outer surface with a plurality of low fins 21 that are helically arranged on the outer surface of said tube 2 according to a first angle of advancement α. This angle of advancement α is generally less than 80°, and preferably between 15° and 60°, more preferably between 20° and 45°.

A further peculiar characteristic of the shell and tube longitudinal flow heat exchanger 1 according to the present invention is given by the fact that the low fins 21 have a profile interrupted by helical grooves 22 having a second angle of advancement β, with α≠β.

To illustrate the characteristics of the tube 2 more clearly, in FIGS. 6a and 6b the fins and the grooves have been schematically represented separately. Also with reference to FIG. 11a , a first machining phase of the tube 2 allows the base of the tube to be lowered (by an amount h) and raised (by the same amount h) so as to produce a corrugated profile having helical fins 22 and corresponding helical grooves with angle of advancement β, as shown in FIG. 6b . The height h with respect to the base profile is preferably less than 0.5 mm.

With reference now to FIG. 11b , a second machining phase of the tube 2 allows the final fin 21 to be obtained by lowering (by an amount H) and raising (by the same amount H) the corrugated profile of the tube 2 of FIG. 11a according to a helical machining operation with angle of advancement α (see FIG. 6a ).

The final structure of the fin 21, in terms of height and of number of interruptions, will thus depend on the composition of the two deformations, in particular on the amounts h and H, and on the angles α and β. When the relative angle between α and β is close to 0° the maximum increase in the height of the fin 21 will be obtained, while when it is close to 90° the maximum number of interruptions on the profile of the fin 21 due to the grooves 22 will be obtained.

With regard to the “shape” of the fin 21, this can be chosen at will according to needs. FIGS. 7a-7c show some possible sections of the fin 21, without being in any way limited to these embodiments.

With reference to FIG. 8, in a preferred embodiment of the shell and tube longitudinal flow heat exchanger 1 according to the present invention, the tubes 2 are provided with finned portions 20 alternated by smooth portions 200. In this way, also with reference to FIG. 15, stable positioning of the baffle 102 will be facilitated.

With reference to FIGS. 9 and 10, there will now be described a first embodiment of a method for manufacturing a finned tube 2 provided on at least one part of its outer surface with a plurality of low fins 21. These fins 21 are helically arranged on said outer surface with a first angle of advancement α and have a profile interrupted by helical grooves 22 having a second angle of advancement β.

The method according to the invention is carried out using a machine 3 comprising a working assembly 30 and at least one support assembly 40. The first working assembly 30 comprises a first rotating finning/grooving tool 32 and a second rotating finning tool 31 mounted in sequence on the same driving axis 33. The support assembly 40 comprises two smooth surface cylindrical guides 34 and 36, the purpose of which is to maintain the tube 2 in position during machining, supporting the thrust load of the working assembly 30.

The first rotating finning/grooving tool 32 is provided with a first helical working profile symmetrical to the helical grooves 22 to be generated on the outer surface of the tube 2 and that has a first angle of advancement α₁.

The second rotating finning tool 31 is provided with a second helical working profile symmetrical to the low fins 21 to be generated on the outer surface of the tube 2 and that has a second angle of advancement α₂, with α₂≠α₁.

The method according to the invention comprises advancing the tube 2 on a plane defined by the support assembly 40 and forming a first fin/groove 22 on said tube 2 by means of the first rotating tool 32. Advantageously, the fin/groove 22 has a depth preferably ≤0.5 mm to limit hardening of the material.

Immediately after the first fin/groove 22 has been formed, a second fin 21 (main fin) is formed on said tube 2 by means of said second rotating finning tool 31. The height of said second main fin 21 is greater than the height of said first fin 22, even if it is normally less than 2 mm.

As explained above, depending on the relative angle between said first angle of advancement α₁ and said second angle of advancement α₂ it is possible to obtain a greater or lesser height of the main fin 21 and a greater or lesser number of its interruptions by the groove 22.

With reference to FIGS. 12 and 13, in a second embodiment of a method for manufacturing a finned tube 2 according to the present invention, a plurality of low fins are formed both on its outer surface and on its internal surface.

In this case, the method according to the invention is carried out using a machine 5 comprising a first working assembly 50 and a support assembly 70, similar to the first working assembly 30 and to the support assembly 40 described previously. With regard to the outer part of the tube 2, machining takes place in the same manner as described previously.

The machine 5 also comprises a second working assembly that is adapted to produce the internal fins of the tube 2. The internal fin is obtained by means of a finning tool 61 with profile symmetrical to what is to be obtained on the internal surface of the tube 2. The tool 61 is inserted into the tube and is “actuated” by the pressure exerted by the first 32 and second 31 rotating tool on the tube 2 resting on the smooth surface cylindrical guides 71 and 72 of the support assembly 70. This causes a reduction in the internal diameter of the tube 2, which is thus finned by the internal tool 61.

The internal fin has a wrap angle contrary to that of the external fin 21 so as to prevent the external or internal tool from binding. Angle of advancement, height of fin and density of fins on the internal part are obtainable in the ranges known in the state of the art.

In brief, the method described uses two profiled tools for external (or external and internal) cold forming of a tube made of low or high alloy steel. This configuration allows high productivity, preventing the otherwise frequent risks of damage/breakage of the tools and reducing to a minimum the complexity of the mechanical apparatus employed. It is also suitable for machining alloy steels, such as copper-nickel, stainless, duplex, titanium steels, which are critical for many alternative methods, as known from the state of the art.

Based on the description above, it has been seen how the method for manufacturing a finned tube, a finned tube thus obtained, and a heat exchanger, in particular a shell and tube longitudinal flow heat exchanger according to the present invention, achieve the intended aims and objects.

On the basis of the description provided, other characteristics, modifications or improvements are possible and evident to a person skilled in the art. These characteristics, modifications and improvements should therefore be considered part of the present invention.

In practice, the materials used, the dimensions and contingent shapes can be any according to requirements and to the state of the art. 

1. A shell and tube longitudinal flow heat exchanger comprising a containment casing within which a first fluid can flow substantially parallel to the longitudinal axis of said casing, said containment casing accommodating in its interior a bundle of tubes substantially parallel to one another and parallel to the longitudinal axis of said casing and a plurality of grid-shaped baffles substantially transverse to the longitudinal axis of said casing supporting said tubes, said bundle of tubes being adapted to the flow of a second fluid therein, characterized in that said tubes are provided on at least a part of their outside surface with a plurality of low fins, said low fins being helically arranged on the outer surface of said tubes with a first angle of advancement α and having a profile interrupted by helical grooves having a second helical angle of advancement β, with α≠β.
 2. A shell and tube longitudinal flow heat exchanger according to claim 1, characterized in that said low fins have a height H between 0.5 and 1.5 mm.
 3. A shell and tube longitudinal flow heat exchanger according to claim 1, characterized in that said first advance angle α is 20°≤α≤≤45°.
 4. A shell and tube longitudinal flow heat exchanger according to claim 1, characterized in that the relative angle between said first angle of advancement α and said second angle of advancement β is between 30° and 60°.
 5. A shell and tube longitudinal flow heat exchanger according to claim 1, characterized in that said tubes are provided on their internal surface of a plurality of low fins.
 6. A shell and tube longitudinal flow heat exchanger according to claim 1, characterized in that said tubes are provided with finned portions alternated by smooth portions.
 7. A method for manufacturing a finned tube using a machine comprising a working assembly and at least one support assembly, said first working assembly comprising a first rotating finning tool and a second rotating finning tool mounted in sequence on the same driving axis, said first rotating finning tool being provided with a first helical working profile having a first angle of advancement at and said second rotating finning tool being provided with a second helical working profile having a second angle of advancement α₂, with α₂≠α₁, the method comprising advancing said tube on a plane defined by said support assembly, forming a first fin on said tube by means of said first rotating finning tool, forming a second fin on said tube by means of said second rotating finning tool, the formation of said second fin being immediately subsequent to the formation of said first fin, the height of said first fin being lower than the height of said second fin.
 8. A method for manufacturing a finned tube according to claim 7, characterized in that the relative angle between said first angle of advancement at and said second angle of advancement α₂ is between 30° and 60°, and in that the height h of said first fin is ≤0.5 mm, and the height H of said second fin is ≤2 mm.
 9. A finned tube for heat exchangers, in particular for a shell and tube heat exchanger, obtained by a method for manufacturing the finned tube using a machine comprising a working assembly and at least one support assembly, said first working assembly comprising a first rotating finning tool and a second rotating finning tool mounted in sequence on the same driving axis, said first rotating finning tool being provided with a first helical working profile having a first angle of advancement at and said second rotating finning tool being provided with a second helical working profile having a second angle of advancement α₂, with α₂≠α₁, the method comprising advancing said tube on a plane defined by said support assembly, forming a first fin on said tube by means of said first rotating finning tool, forming a second fin on said tube by means of said second rotating finning tool, the formation of said second fin being immediately subsequent to the formation of said first fin, the height of said first fin being lower than the height of said second fin.
 10. A finned tube for heat exchangers according to claim 9, characterized in that it is provided on at least a part of its outside surface with a plurality of low fins, said low fins being helically arranged on the outer surface of said tube with a first angle of advancement α and having a profile interrupted by helical grooves having a second helical angle of advancement β, with α≠β, said first angle of advancement α being 15°≤α≤60°, the relative angle between said first angle of advancement α and said second angle of advancement β being between 30° and 60°, said low fins having a height H between 0.5 and 1.5 mm. 